Shiurim about zemirot and piyyutim by Rabbi Alan Haber
Shiurim about zemirot and piyyutim by Rabbi Ariel Rackovsky
The Piyut and Tefila Site (formerly Piyut.org.il): One of the best resources available online (mainly in Hebrew with English content for selected piyyutim), including recordings of piyyutim in various traditions. Much of the content about Piyyutim on my site was adapted from resources on Piyut.org. This site was especially helpful in preparing my piyyut study sessions and, later, translations.
Piyut North America: English content about piyyutim with selected translations available on the Resources page. Look for my name as the translator for some of their piyyutim. (Most of my contributions were written with support from Dr. Jonathan Decter at Brandeis)